They are called “homegrown terrorists,” citizens of Western countries highly prized by Islamist militant groups because they can move across borders and carry out attacks easier than people from Middle East and South Asian countries closely identified with terrorism.

Two such people – one Canadian and one Australian – are believed to have been involved in the July bus bombing in Bulgaria that killed five Israeli tourists and a Bulgarian driver, according to Bulgarian investigators. Bulgarian Interior Minister Tsvetan Tsvetanov said the two were members of the Lebanese Shiite Muslim militant group Hezbollah, which is in turn linked to Iran.